During gas turbine engine operation, hot section components such as combustors, turbine blade and vane airfoils, turbine frames, and exhaust nozzles are subject to oxidizing and corrosive, high temperature combustion effluent gas. Because these components often are subjected concurrently to high magnitude thermally and mechanically induced stress, the art has developed a variety of techniques in the design and manufacture of these components to ensure maintenance of structural and metallurgical integrity throughout the operating range of the engine. For example, components typically are manufactured from material compositions such as nickel- and cobalt-base superalloys having desirable properties at elevated, operating range temperatures. In the case of turbine airfoils, the selected alloy generally is formed by casting. For enhanced high temperature strength, grain structure advantageously may be controlled during solidification of the casting to produce a directionally solidified or single crystal structure, thereby providing greater strength for a given alloy composition.
In addition to component strength enhancement by selection of alloy composition and control of the casting process, both internal and external cooling schemes are employed extensively to maintain component temperatures below critical levels. Tailored film cooling of external surfaces and sophisticated turbulent flow cooling of serpentine shaped internal cavities in the cast airfoils are routinely utilized in advanced gas turbine engine designs respectively to decrease the thermal energy input to the component and reduce the temperature rise thereof. Despite efforts to optimize these varied approaches, both alone and in combination, advanced gas turbine engine efficiency is limited by the inability of the hot section components to achieve acceptable operating lives under increased mechanical and thermal loading.
An additional method employed by those skilled in the art of gas turbine engine design is the use of a relatively thin ceramic insulative outer layer on surfaces exposed to the effluent gas flow. The ceramic coating facilitates component operation at greater operating temperatures. This coating, generally referred to in the industry as a thermal barrier coating ("TBC"), effectively shields the metallic substrate of the component from temperature extremes. By reducing the thermal energy input to the component, higher combustion effluent gas temperatures and/or more efficient use of cooling flows are realized with a resultant increase in engine operating efficiency.
Conventional ceramic coatings are prone to delamination at or near the ceramic/substrate interface due to differences in coefficients of thermal expansion between the relatively brittle ceramic and the more ductile superalloy substrate. The ceramic may spall or separate from the component surface. This failure mechanism is aggravated and accelerated under conditions of thermal cycling inherent in gas turbine engine operation. In order to prevent premature failure of the ceramic, methods of providing strain tolerant ceramic coatings have been developed. Certain moderate service applications employ porous or pre-cracked ceramic layers. In more harsh operating environments, such as those found in advanced gas turbine engines, the art exploits strain tolerant open columnar ceramic crystal or grain microstructures, such as those described in U.S. Pat. No. 4,321,311 to Strangman, the disclosure of which is herein incorporated by reference. These columnar grain microstructures have a generally parallel grain orientation and are disposed in a normal direction, perpendicular to the surface of the substrate. They are considered to provide improved strain tolerance due to the segregated nature of the columnar grains which form intercolumnar gaps therebetween.
Substantial attention also has been directed to the use of an intermediate or bond coat layer disposed between the substrate and the ceramic layer. The bond coat employs a composition designed both to enhance the chemical bond strength between the ceramic topcoat and metal substrate as well as to serve as a protective coating in the event of premature ceramic topcoat loss.
There are presently two primary classes of bond coat compositions conventionally employed in multilayered TBC systems of this type. One type of metallic bond coat typically specified by gas turbine engine designers is referred to as MCrAlY alloy, where M is iron, cobalt, nickel, or mixtures thereof The other major constituents, namely chromium, aluminum and yttrium, are represented by their elemental symbols. As used herein, the chemical symbol "Y" signifies the use of yttrium as well as other related reactive elements such as zirconium, lanthanum, and mixtures thereof. A conventional MCrAlY bond coat is described in U.S. Pat. No. 4,585,481 to Gupta et al., the disclosure of which is herein incorporated by reference. In coating a superalloy substrate, the MCrAlY bond coat first is applied to the substrate by a method such as physical vapor deposition ("PVD") or low pressure plasma spraying.
The MCrAlY class of alloys are characteristically very resistant to oxidation at the elevated temperatures experienced by hot section components due to their ability to form a thin adherent protective external film of aluminum oxide or alumina. As used herein, the term "alumina" signifies predominantly aluminum oxide which may be altered by the presence of reactive elements to contain, for example, yttrium oxide or zirconium oxide. In addition to providing protection, the alumina film also provides a chemically compatible surface on which to grow the insulative ceramic topcoat. As known by those having skill in the art, the ceramic topcoat most commonly employed is zirconium oxide or zirconia, either partially or fully stabilized through the addition of oxides of yttrium, magnesium, or calcium. Conventional open columnar structured stabilized zirconia is grown on the alumina film by PVD in which the component to be coated is rotated at a constant rate in a ceramic vapor in a vacuum chamber. This coating system generally is considered to exhibit improved integrity under cyclic thermal conditions over ceramic coatings disposed directly on the metallic substrate, thereby providing the intended insulative protection to the underlying article over an extended period.
Another type of metallic bond coat routinely specified by those skilled in the art includes a class of materials known as aluminides. These are popular compositions for gas turbine engine components and include nickel, cobalt, and iron modified aluminides as well as platinum modified aluminides. Generally, aluminides are intermediate phases or intermetallic compounds with physical, chemical, and mechanical properties substantially different from the more conventional MCrAlY bond coats. Some aluminide compositions are known to be usefull coatings in and of themselves for protecting iron-, cobalt-, and nickel-base alloys from oxidation and corrosion; however, some aluminides may be used as bond coats for ceramic topcoats in TBC systems.
The aluminide-based TBC system is similar to the MCrAlY-based TBC system insofar as the aluminide bond coat is first formed on the substrate surface by conventional diffusion processes such as pack cementation as described by Duderstadt et al. in U.S. Pat. No. 5,238,752 and Strangman in published U.K. Patent Application GB 2,285,632A, the disclosures of which are herein incorporated by reference. According to this method, aluminum from an aluminum halide gas in the pack mixture reacts and interdiffuses with the substrate surface over time at elevated temperature. Strangman discusses production of aluminide bond coats, for example, by reacting a nickel-, iron-, or cobalt-base superalloy article substrate with an aluminum rich vapor at elevated temperature. Strangman refers exclusively to the term "diffusion aluminide" as characteristic of the resultant bond coat. This characterization accurately corresponds to the method of aluminide bond coat production, namely by diffusion. As a result of the diffusion method, the aluminide bond coat contains nickel, iron, or cobalt from the substrate of the component being coated, depending on the primary constituent of the superalloy substrate. Further, many of the base alloying elements of the substrate are also contained in the reaction product aluminide forming on the article surface. The aluminide coated article has a surface composition which readily forms a protective alumina film when oxidized. A columnar ceramic topcoat of conventional composition and structure, as described hereinabove, completes the TBC system.
A new metallic bond coat has recently been developed which is contemplated to overcome inherent limitations of conventional MCrAlY and aluminide bond coats. An MAlY bond coat, where M is nickel, cobalt, iron, or combinations thereof and Y is yttrium or other reactive element, is disclosed in U.S. patent application Ser. No. 08/597,841, the disclosure of which is herein incorporated by reference. A TBC system incorporating an MAlY bond coat, alumina layer, and columnar ceramic topcoat disposed on a nickel- or cobalt-base superalloy article, is considered to be less prone to degradation and failure than TBC systems utilizing the conventional bond coats discussed above. The bond strength or adherence between the MAY bond coat and alumina film is enhanced over conventional aluminide and MCrAlY bond coats by controlling the weight percent of the constituents. Further, by specifically excluding chromium from the bond coat, diffusional stability of the chromium-free MAlY bond coat is significantly improved over conventional MCrAlY bond coats. As a result, the MAlY bond coat provides a substantial reduction in diffusion of substrate alloy constituents through the MAlY bond coat, and maintenance of a strong MAlY/alumina bond resistant to degradation as a function of time at elevated temperature, with a concomitant enhancement in ceramic topcoat integrity. Yet further, the MAlY/alumina bond is stronger than that of a conventional modified aluminide/alumina bond. In addition, the growth rate of the alumina film is reduced by the presence of yttrium or other reactive element and the combined effect exhibits improvement over conventional aluminide-based TBC systems.
While conventional columnar ceramic topcoats have proven beneficial in use, they exhibit certain characteristics which inherently limit their operational performance and life regardless of the bond coat composition. For example, since conventional columnar ceramic topcoats have a grain orientation direction which extends generally perpendicularly from the underlying substrate, the grains are relatively inelastic in the normal direction and exhibit poor impact strength, resulting in chipping when contacted by high speed sand grains and other foreign particulate matter ingested or released upstream during engine operation. Erosion of the ceramic topcoat can also be a problem due to the cantilevered orientation of the columnar grains which are bonded to the alumina layer over a small surface area relative to their volume. Accordingly, the normal columnar ceramic grains are weak in a tangential or parallel direction to the substrate, resulting in brittle spalling failure of the ceramic topcoat. The underlying alumina layer, bond coat, and metallic substrate are also subject to environmental attack since the intercolumnar gaps or interstices between adjacent columnar grains afford numerous open paths for penetration of corrosive salts and other detrimental constituents of the combustion effluent. Conventional columnar ceramic topcoats also add weight to the components, which can be problematic especially for high speed rotating components such as turbine blades where overall weight and balance are critical parameters.
Service lives of the coated articles are limited by the integrity of the TBC. Any substantial chipping, spalling, erosion, or delamination of the ceramic topcoat is cause for concern. The consequences of TBC system failure are tangible and costly. Firstly, thermal operating margin must be factored into the design of the gas turbine engine to preclude overtemperature and failure of hot section components with degraded TBC. By limiting combustion parameters to less than stoichiometric, the realizable efficiency of the engine is reduced with increase in fuel consumption as well as levels of unburnt hydrocarbons and other pollutants. Further, baseline engine operating parameters are premised on the existence of uniform ceramic topcoats, and ceramic topcoat life is often significantly less than underlying component life. This means engines must be removed from service for maintenance at predetermined intervals, based, for example, on operating hours or thermal cycles. Combustor, turbine, and exhaust modules are disassembled and the coated articles removed, stripped, inspected, and recoated. Significant costs are attributable to aircraft and engine unavailability. Further, substantial direct costs are associated with labor, tooling, and materials required to remove, recoat, and reinstall the affected articles. Yet further, unscheduled engine removals are forced whenever borescope inspection of the internal configuration of the engine reveals TBC system degradation beyond predetermined field service limits, further disrupting operations and increasing support costs.